[MEncoder-users] Mencoder quality settings ...

[Moritz Barsnick]

Hi,

On Thu, Jan 03, 2008 at 20:58:24 +0100, Leopold Palomo Avellaneda wrote:
> I would like to convert it to using another codec (for example xvid or x264) 
> preserving the _same_ quality (in terms of taking one frame from both and not 
> finding any difference). It's something like I have a bunch of files and I 
> compress it using bz2 or zip. I know that I always will have the original. 
> Some kind of lossless compression.

Let me take a mathematical and logical approach here:

> I think that it's impossible because when I decode a frame and I
> recode it it cannot be equal to the original.

It can, if the codec used when recoding is lossless. Not many such
codecs come to mind, I think snow and hufyuv are candidates. As
mentioned in the other answers, there _are_ settings for other more
common codecs making them "nearly" lossless, even possiby at a bitrate
possibly lower than your original.

> But for example, if I have a photo, for example in tiff and I save it
> to jpeg, I can choose the % of quality, understanding that a 100%
> quality jpeg is equal to the original tiff using less space.

This is wrong since JPEG has _no_ lossless option available whatsoever.
JPEG2000 e.g. on the other hand does. I understand what you're trying
to say though, but the example is wrong. ;-)

> In my case I don't care about the disk space totally but yes to have a 
> conversion to another more efficient codec.

Your problem is you want both lossless and more efficient than MPEG2.
At the same time your "original" is already MPEG2 and thereby lossy. If
you had captured from that "home video device" to something less lossy
or non lossless, your next (lossy) recoding step would have the chance
of being closer to this "absolute" original.

> So, if I don't want to look in to the bitrates, etc, there's some way to 
> convert one stream in one format to another preserving the _same_ image 
> quality but less space?

Again, in the chain:
real original -> MPEG2 -> next format
the next format will statistically always be worse (further from the
real original) unless it uses a lossless codec.
If you replace that chain (which you might not be able to) with
real original -> new format
you have the chance of using less space and even of higher quality.


Now, apart from that:
I totally get your point. I sometimes also think something along this
line:

"Okay, I have this video here which is already half-bad, and I must
recode it to a different format. With what settings can I get a
satisfactory low bitrate result, considering that my original format
was already quite bad."

Not quite the same as your issue, but along the same lines IMO.

Regards,
Moritz